Surface wave acoustic signal channels are often used as filters, for example in the IF sections of commercial television receivers. A typical configuration employs a sending interdigital transducer and a receiving interdigital transducer upon a substrate of piezoelectric material. The primary propagation direction of the sending transducer and the primary reception direction of the receiving transducer are parallel, but are usually offset from each other to avoid bulk mode coupling. A multi-strip coupler is employed to transfer the acoustic signal wave laterally from the propagation path to the reception path.
The transducers are also coupled, however, by a diffracted surface wave, which does not travel along these primary paths and is not transferred in the normal way by the multi-strip coupler. This diffracted wave travels in various directions, including a direct path extending more or less diagonally across the coupler to the receiving transducer. Such direct coupling is undesirable in a filter or any other frequency-selective device, because the frequency response of the receiving transducer to the diffracted wave is different from its frequency response to the primary wave. Neglecting diffraction effects, the theoretically expected graph of output voltage over signal frequency for the primary transducer system comprises a passband and rejection bands at frequencies both above and below the passband. The effect of the diffracted wave, however, is to increase the response amplitude within the rejection bands. See the article by the present inventor entitled: "Spurious Coupling Between Length-Weighted Transducers Acoustically Connected By Means of a Multistrip Coupler," in Electronics Letters of May 16, 1974, Vol. 10, No. 10, pp. 172-73.
Often at least one of the transducers is apodized (fingerlength-weighted) to enhance its frequency selectivity. Some of the fingers in such a transducer are relatively short. Since diffraction effects are more pronounced when shorter transducer fingers are employed, the problem of diffraction coupling is even more serious in devices employing one or more apodized transducers.
It would be desirable to prevent or compensate for diffraction coupling, and thereby obtain a frequency response closer to that which is theoretically predicted for the primary wave alone. One suggestion in the prior art is to deposit a surface acoustic wave barrier of dampening or reflecting material over the multi-strip coupler. This approach is disclosed in the present inventor's U.S. Pat. No. 4,004,254, as well as in the above-cited technical article.
The effects of diffraction can also be minimized by enlarging the aperture (i.e. breadth) of the surface wave transducers employed, because such enlargement entails longer transducer finger lengths, which are less susceptible to diffraction effects. But this alternative is quite costly in terms of substrate area when surface wave devices are fabricated upon substrates of monocrystalline YZ cut or 128.degree. cut lithium niobate (LiNbO.sub.3). When these substrate materials are used for TV filters, therefore, it is customary to keep the maximum aperture size down to a value of 15.lambda..sub.o, where .lambda..sub.o is the center wavelength of the filter passband. In filters with a transducer aperture that size, diffraction effects are significant. The total diffraction and other spurious response in such devices has been measured at about 40 dB below the level of the first order response. This is far from negligible.